This invention is used to evaluate the performance of primarily ultra high speed digital circuits; although circuits built with any speed logic can be tested. Conventional evaluation techniques are either limited in speed and cannot be used to test the fastest available logic at rated speed or if operated at the desired speed do not provide for precise monitoring of the logic performance. The circuit described here operates at state-of-the-art speeds and gives exact measurements of outputs from the digital unit under test.
A technique which is often used to test digital arithmetic and signal processor circuits is to apply a test signal from an analog signal generator through an analog-to-digital (A/D) converter to the circuit under test. The digital output of the circuit is then applied to a digital-to-analog (D/A) converter so that the equivalent analog output signal can be observed on an oscilloscope or some other suitable recording device. The fundamental speed limitation of this test procedure is the A/D converter. Available 16-bit word plus 16-bit word digital adder circuits perform additions in 10 nsec or less. This addition speed would require a 16-bit output word A/D converter with a sampling rate of at least 1/(10 nsec)=100 MHz, well beyond the state-of-the-art conversion which is approximately 16 bits at 200 KHz. Even if an A/D converter was available to generate the digital input at the desired speed, the D/A converter will eventually limit the maximum speed and both converters have inaccuracies which mask the performance of the circuit under test.
Another common technique to test digital arithmetic is to use a "digital data simulator" to input digital words to the circuit under test from some sort of digital memory. By inputing words from a controlled digital source the inaccuracies of the A/D are eliminated. Commercial data simulators which are nothing but digital memories that can be stored with desired digital data are available to generate sequences of digital words for input to the arithmetic unit. However, the commercial testers are often time limited in maximum speed of operation, and, more importantly, provide no observation of the test unit outputs. A D/A converter is normally used to monitor the output. Another possibility for monitoring is to observe single bits of the output words on an oscilloscope, a technique that provides little information about the overall circuit operation.